按下按钮时JList的更新

Question

I was trying to update the contents of a JList when a button was pressed. So, I cleared the list model, then cleared the list and then proceeded to add new values to the list. Here is the stripped code:

testList.java

public class testList extends javax.swing.JFrame {

    private Thread t;
    public DefaultListModel model;
    public boolean first = true;

    public testList() {
        model = new DefaultListModel();
        initComponents();
        this.centre(this);
    }

    public static void centre(javax.swing.JFrame f) {
        Dimension us = f.getSize(), them = Toolkit.getDefaultToolkit().getScreenSize();
        int newX = (them.width - us.width) / 2;
        int newY = (them.height - us.height) / 2;
        f.setLocation(newX, newY);
    }

    class updateList implements Runnable {

        public void run() {
            tmp.getTheList();
            model.clear();
            ouputList.removeAll();

            for (int i = 0; i < tmp.returnList.size(); i++) {
                model.addElement(tmp.returnList.get(i));

            }
            if (first) {
                chList.setModel(model);
            }

        }
    }

    private void initComponents() {
    // generated by NetBeans 6.9
    }

    private void buttonActionPerformed(java.awt.event.ActionEvent evt) {
        t = new Thread(new updateList(), "List Updater");
        t.start();
    }

    public static void main(String args[]) {

        tmp = new aC();

        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                new chapList().setVisible(true);
            }
        });
    }

    static aC tmp;

    private javax.swing.JButton button;
    public static javax.swing.JList outputList;
    private javax.swing.JScrollPane jScrollPane1;
}

ac.java

public class aC extends testList {

    ArrayList returnList = new ArrayList();

    void getTheList() {
        returnList.clear();
        generateList();
    }

    void generateList() {
    // populate returnList with random values of random size using returnlist.add()
    }
}

The problem I am facing is that when the list created for the first time, it updates the JList. When the button is pressed again, the JList only gets updated sometimes. And for further presses of the button nothing is displayed in the JList.

Could someone help me figure out what is causing this problem? Thanks.

Answer 1

Your core problem is probably related to updating the Swing GUI from a thread that isn't the AWT-EDT .

You might want to read about and/or look into using SwingWorker (shipped with Java 6 and also available for download for use with earlier versions of Java.)

Alternatively, have a look at this approach:

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;

public class BackgroundWorkerFrame extends javax.swing.JFrame {
    public BackgroundWorkerFrame() {
        initComponents();
        jList.setModel(new DefaultListModel());
    }

    private void jButtonGoActionPerformed(ActionEvent evt) {                                          
        Thread t = new Thread(new WorkerRunnable());
        t.start();
    }                                         

    public class WorkerRunnable implements Runnable {
        public void run() {
            System.out.println("Working hard...");
            sleep(1000);
            ArrayList<Integer> list = new ArrayList();
            for (int i = 0; i < 5; i++) {
                list.add((int) (Math.random() * 100));
            }
            System.out.println("Updating GUI...");
            SwingUtilities.invokeLater(new UpdateRunnable(list));
        }
    }

    public class UpdateRunnable implements Runnable {
        private final ArrayList<Integer> list;
        private UpdateRunnable(ArrayList<Integer> list) {
            this.list = list;
        }
        public void run() {
            DefaultListModel model = (DefaultListModel) jList.getModel();
            model.clear();
            for (Integer i : list) {
                model.addElement(i);
            }
        }
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        jButtonGo = new JButton();
        jScrollPane = new JScrollPane();
        jList = new JList();

        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        jButtonGo.setText("Go");
        jButtonGo.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                jButtonGoActionPerformed(evt);
            }
        });
        getContentPane().add(jButtonGo, BorderLayout.PAGE_START);

        jScrollPane.setViewportView(jList);

        getContentPane().add(jScrollPane, BorderLayout.CENTER);

        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        setBounds((screenSize.width-309)/2, (screenSize.height-338)/2, 309, 338);
    }// </editor-fold>

    public static void sleep(long ms) {
        try {
            Thread.sleep(ms);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
            Thread.currentThread().interrupt();
        }
    }

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new BackgroundWorkerFrame().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify
    JButton jButtonGo;
    JList jList;
    JScrollPane jScrollPane;
    // End of variables declaration
}

Answer 2

One thing you are doing is creating a new Thread and then making changes to your GUI component outside of the EventDispatchThread. This is generally not a great idea. Try running the updateList() in place, which will be on the EDT, as the button events are handled in that thread.